Many types of opening/closing devices for a double flap door based on the articulated quadrilateral principle are known to the art; their structural elements and operation will not be described herein because they are per se known.
It will merely be recalled that in opening/closing devices of the prior art, the two parts of the flap door are hinged to each other and, in the open position, fold over one another; the hinge between the two parts of the flap door forms one of the articulation points of the articulated quadrilateral, whereof one of rods is fixed to the bottom part of the door.
The fact that the two parts of the flap door are constrained to each other implies that:                in the open position, the two parts of the flap door assume a (more or less) obligatory position;        since the top part of the flap door is hinged to the edge (normally horizontal) of the furniture item, the sides of the two parts of the flap door hinged to each other must be horizontal and in any case parallel to the edge of the furniture item.        
All this imposes aesthetic and functional constraints and/or limitations on the designers and technicians who design furniture.